Power steering gear



Feb. 3, 1953 F. w. DAVIS 2,627,187

POWER STEERING GEAR Filed Oct. 20, 1951 3 @4220 M (ZZZ/5d Patented Feb. 3, 1953 UNITED STATES PATENT OFFICE POWER STEERING GEAR Francis W. Davis, Belmont, Mass.

Application October 20, 1951, Serial No. 252,246

8 Claims.

This invention relates to improvements in power steering mechanisms such as are used on motor-driven land vehicles, and more particularly to improvements in the steering mechanism illustrated and described in my U. S. Patent No. 2,213,271, granted September 3, 1940. The mechanism shown in the patent includes a worm fixed on the steering shaft which carries the steering wheel and on which is mounted the inner member of a sleeve valve to control the supply of power fluid to the steering motor. The worm in the patented mechanism meshes with a gear section operatively connected to linkage which deflects the vehicle wheels to determine the course of the vehicle. According to the present inven tion the worm is positively held against any axial movement relative to the housing or frame in which it is mounted, and the worm can be and preferably is of the hour-glass type, that is, with a diameter varying from a minimum at its mid point to a maximum at both extremities. To provide for the operation of the control valve, the steering shaft, on which is mounted one of the valve members, projects into a longitudinal bore in the worm, the portion of the shaft within the bore being screw-threaded and in threaded engagement with the interior of the worm, whereby any relative rotation between the shaft and worm results in axial movement of the shaft relative to the housing or frame, since the worm is held against such movement. According to the invention, the screw thread which connects the shaft and worm is preferably made with a steep pitch, that is, considerably steeper than the pitch of the thread on the exterior of the worm. This gives reversability of action between the shalt and worm. The effectof this is to increase the distance of axial movement of the shaft resulting from a given angular movement thereof relative to the worm. This increase in axial movement has a corresponding effect on the operation of the control valve because the amount of opening or closing of the valve ports depends on the amount of axial movement of the shaft. This means increased sensitivity of response in the operation of the valve.

For amore complete understanding of the inventicn reference may be had to the following description thereof and to the drawings of which Figure .1 is. a side elevation of a mechanism embodying the invention, portions being broken away to show the moving parts in section; and

Figure 2 is a section on line 2--2 of Figure 1.

The drawing illustrates a steering mechanism which is mounted in the frame or housing ll].

valve indicated at i i.

From this frame a steering column 12 extends up to the steering wheel (not'shown). In the steering columnis a steering shaft H! which is rotatable and is also axially movable, the range of axial movement being limited to a few thousandths of an inch. The lower portion of this shaft extends through a hollow worm [6 which, as shown, is of the hour-glass type. The steering shaft ['4 is mechanically connected to the worm is by means of a screw threaded portion [8 which meshes with a corresponding interior thread within the bore of the worm IS. The thread or helix l8 has preferably a steep pitch, a in pitch being illustrated on the drawing. This pitch is considerably greater than'the pitch of the exterior threads of the worm l6. Any relative rotation between the worm I6 and the shaft results in axial movement of the latter since the worm is positively held against any axial move ment by means of suitable bearings such as the roller bearings 2E5 shown in the drawing. The worm meshes with a cam roller 22 carried by a gear sector member 24. This member is fixed on a rock shaft 25 which is journalled in the housing ill and is operatively connected through suitable linkage to the vehicle wheels (not shown) The member 24, in addition to the cam roller 22 which meshes with the worm Hi, has a gear sector to which meshes with a rack 32 on the end of a piston rod 34. The rod 34 extends from a piston 36 reciprocable in a fluid motor cylinder 38. The introduction of fluid under pressure into the cylinder 38 results in pressure against the piston 35 tending to move it in one direction or the other depending on which end of the cylinder receives the pressure fluid.

The pressure fluid is supplied to the epposit-e ends of the cylinder by means of suitable pipes or ducts t9 and 52 which lead- 'from a control This valve is preferably similar to that described and illustrated in my Patent No. 2,213,271 and may be connected by suitable pipes or ducts to a pump, reservoir and Icy-pass relief valves in the manner shown and described in my Patent No. 2,007,423.

Valve it includes grooves it and '28 in the interior of a tubular member 553 which may be a part of the frame or housing :6. Cooperating with the grooves 36 and to are grooves 52 on theexterior of the sleeve 54% which is carried on the shaft M but does not rotate. The sleeve is confined between ball bearing races 56 which in turn bear respectivelyagainst a shoulder 53 on the steering shaftand nut 69 which is screwthreaded on the shaft. Thus the valve sleeve 5i! partakes of any axial movement of the shaft I4 but does not rotate when the shaft rotates. The grooves 52 are constantly in communication with the ducts and 42 and are located between the groove 48 and the grooves 46. The groove 48 is connected to an inlet 6| by which fluid under pressure, preferably oil, is supplied to the valve. The grooves 46- are connected to a return pipe (not shown) which leads back to the pump section or reservoir so that oil can be continuously circulated. When the valve sleeve 54 is in its neutral position as illustrated in Figure 1, the grooves 52 slightly overlap the edges of the grooves 46 and 48 providing clearances through which oil which flows in through the groove 48 can flow freely into both grooves 52 and thence out through the grooves 46. This results in low, balanced pressures in the chambers in the cylinder 38 and in very little load on the pump. When the sleeve 54 is axially moved in one direction or another from its neutral position, the opening between the supply groove 48 and one of the grooves 52 is widened while the opening between the supply groove 43 and the other groove 52 is correspondingly narrowed. Simultaneously the opening between the first groove 52 and the discharge groove is narrowed while the opening between the second groove 52 and the adjacent discharge 46 is correspondingly widened. This results in a building up of pressure in the end of the chamber 35 corresponding to the groove 52 which has been opened to the inlet roove 48.

This unbalanced pressure in the cylinder tends to move the piston and consequently to rock the gear sector member 24 in a direction to rotate the worm is to catch up with the shaft and restore their customary relative position.

Axial movement of the sleeve 55 is yieldingly opposed by a set of two or more preloaded spring pressed devices, one of which is illustrated in Figure 1 of the drawing, these devices being distributed around the shaft l 4. Each device consists of a pair of plungers 62 which are pressed apart by a compressed spring 64. The mutually remote ends of the plungers 62 bear against the ball bearing races 55 and also against shoulders 66 of the casing 16. The springs 64 are constantly under compression so that any movement of either plunger 62' away from its shoulder 66 will have to be caused by sufiicient force to overcome the initial compression of the springs 62. Thus the shaft I4 is held against any axial movement until such force is impressed thereon.

When the steering mechanism is in use, rotation of the shaft l4 results in rotation of the worm l6 and this ordinarily results in rocking movement of the gear sector member 24 so that the vehicle wheels are accordingly deflected. If however the resistance to steering effort exceeds a predetermined magnitude so that worm I6 is prevented from following the rotation of the shaft M, then the shaft rotates relatively to the Worm and thus moves axially by reason of the helix l8 which connects the shaft mechanically to the worm I6. This axial movement of the shaft results in a similar axial movement of the valve sleeve 54 which directs power fluid into the cylinder 38 in such a manner as to cause the piston 36 to rock the gear sector member 24. This rocking movement not only deflects the vehicle wheels but also it acts through the rol1er cam 22 to rotate the worm IS in a direction to restore the shaft I8 to its neutral position relative to the worm. This also restores the valve sleeve 54 to its neutral position and balances the fluid pressure in the cylinder 38.

The steep pitch of the helix I8 which connects the steering shaft l4 with the worm It results in an increased sensitivity in the action of the valve since for a given angle of relative rotation between the shaft and worm there is a greater axial movement on the part of the shaft than there would be if the shaft were fixed to the worm and the worm were allowed to move axially with the shaft.

In case of failure of the power, the mechanism is used as an ordinary hand-steering gear. If road resistance is sufficient to overcome the force of the centering springs 64, the resulting axial movement of the steering shaft [4 relative to the worm I6 is limited by the engagement of a shoulder 70 on the shaft with the upper end of the worm, or by engagement of a nut 12 on the shaft with the lower end of the worm. The clearances between the ends of the worm and the shoulder 70 and nut 12 are less than those between the ball-bearing members 56 and the ends of the valve housing 50, so that the axial thrusts of the steering shaft incidental to handsteering without power are taken by the strongly mounted worm it rather than by the valve casing and ball-bearings.

I claim:

1. In a power steering mechanism, a frame, a hollow worm mounted in said frame for rotation therein, means holding said worm from axial movement relative to said frame, a shaft coaxial with said worm, said shaft having a portion in screw-threaded engagement with the interior of the worm, whereby relative rotation between the worm and shaft results in axial movement of the shaft, a worm-gear sector member mounted on said frame in mesh with said worm for movement in response to rotation of said worm, a fluid motor operatively connected to said gear sector member, a control valve operable by axial movement of said shaft to direct power fluid to said motor, and ducts connecting said control valve to said motor.

2. Mechanism as in claim 1, said hollow worm being of the hour-glass type.

3. Mechanism as in claim 2, said worm-gear sector member being rockably mounted in said frame.

4. Mechanism as in claim 3, said worm-gear sector member having a second gear sector, said fiuid motor including a piston rod, and a gear rack on said piston rod meshing with said sec- 0nd gear sector.

5. Mechanism as in claim 3, said worm-gear sector member including a body member and a roller cam rotatably mounted on the body memher, said roller cam having a screw thread meshing withsaid worm. V

6. Mechanism as in claim 1, the interior screwthread of the hollow worm having a substan-- tially greater pitch than the worm itself.

7. Mechanism as in claim 1, and pro-loaded spring means yieldingly opposing axial movement of said shaft in either direction to said frame from an intermediate neutral-position.

'8. Mechanism as in claim 7, said worm and shaft having stop elements for positively limiting the axial movement'sof the shaft relative to the worm.

FRANCIS W. DAVIS.

N o referencescited. 

